The past tense of spell. Alternative spelling is spelled. Americans may not use this term, as it was almost sacrificed to Webster's mania for making English over in his image in that dictionary. This is why we say that you speak 'American' rather than English. Many words are spelt differently or not used there, except as (sic) mispellings. If you still disagree, visit the Oxford English Dictionary online. I believe the OED is generally conceded to be the final arbiter of disputes concerning vocabulary in English, is it not? They say that I'm right.
They also say it's a tasty little grain that's very good for you.

A grain, in American English. Just because spell check does not correct it does not mean that it is the past tense or past participle of the word SPELL. Correct would be SPELLED. Now if we are talking British English then it could be used that way. However, just because you can't spell doesn't mean you get to use the British English excuse. You have English every year that you're in school and some people just never pick up a damn thing...

I would like to harvest the spelt.

Morons, idiots, and imbeciles spell SPELLED with a T because they see no red line under it.